Various movement or position encoders for sensing linear, rotary or angular movement are currently available. These encoders are generally based on either optical systems, magnetic scales, inductive transducers, or capacitive transducers. Certain of these encoders are designed for making relative, and not absolute, measurements. In such relative position encoders, measurements are typically made by sensing the relative change in position of the scales with respect to a reference position, which requires a continuous sensing of the change in the scale pattern so that repetitions of the pattern can be counted. The relative measurements require that a new reference or zero position be established before every measurement, which makes such devices relatively inconvenient to use.
In general, the rate at which the scales of relative measurement devices can be displaced with respect to each other is limited by the speed of the signal processing which can be accomplished. On one hand, if the scales are displaced too quickly, miscounting can occur. On the other hand, increasing the allowable scale displacement speed entails the use of high frequency signals and sophisticated signal processing circuitry, which substantially increases the cost of the measurement devices.
For optical encoders, a number of relative position systems have been developed. One recent system utilizing fewer parts than most previous systems is disclosed in U.S. Pat. No. 5,909,283, to Eselun. The system described in the '283 patent has a grating scale and readhead including a point source (laser diode in readhead), a Ronchi grating or holographic element, and a photodetector array. As described, the point source results in interference fringes having a spacing equal to that of the scale. The interference fringe light is transmitted through the Ronchi grating or holographic element to the photodetector array. The photodetector array is arranged to derive four channels of quadrature signals from the transmitted fringe light. In addition to being a relative position system, another drawback of the system described in the '283 patent is that the resulting encoder is of a size that is relatively large or prohibitive for a number of applications.
Another type of relative position optical encoder is disclosed in U.S. Pat. No. 4,733,071, to Tokunaga. The system described in the '071 patent has a code member scale, and an optical sensor head comprising an optical fiber tip light emitter and two optical fiber tip receptors closely arranged along the code member measuring axis. The optical sensor head is rotated (yawed) to adjust phase difference between the two optical fiber tip receptors. However, the accuracy of the resulting encoder is relatively crude.